An inverter is used as a power supply for a motor to generate a desired output and to rotate at a desired rotation speed. The inverter rectifies and converts power from a line power source to DC power and further converts the DC power to AC power having a voltage and a frequency suitable for driving the motor, to supply the converted power to the motor. A resistor for dissipating regenerated energy from the motor and the DC voltage smoothing capacitor are connected to the DC circuit section of the inverter. While variable speed operation of the motor entails regenerative operation during deceleration, connection of a mechanical load having a large moment of inertial to the motor or rapid deceleration of the motor steeply raises the voltage across the smoothing capacitor owing to regenerated energy. In particular, when the power rating of the resistor is insufficient, the voltage rise phenomenon is considerable. In order to prevent the device from damage due to the overvoltage of the DC circuit section during the regenerative operation of the motor, a motor control device is widely used that is equipped with an overvoltage protector that detects the voltage across the smoothing capacitor and automatically stops the deceleration operation when the detected voltage exceeds a predetermined value.
In such a motor control device, methods of suppressing the amount of regenerated energy returned to the inverter by increasing motor loss have been known as a way to achieve rapid deceleration without causing an overvoltage state of the DC circuit section of the inverter. For example, a method has been known in which the amplitude of voltage applied to the motor is amplified during deceleration to increase the current of the motor and the corresponding magnetic flux of the motor, whereby copper loss and iron loss of the motor is increased to increase the motor loss (see Patent Document 1).
In the technology disclosed in Patent Document 1, however, a current overshoot may occur immediately after the amplitude of voltage applied to the motor is amplified. There is a document that shows a measure against the current overshoot (see Patent Document 2). The document also discloses a technology of a rapid deceleration control of the motor, in which the rapid deceleration control is temporarily stopped to protect the motor and the inverter when a current overshoot, i.e., an overcurrent is detected. This suppresses amplification of the voltage amplitude during the rapid deceleration control of the motor, thereby preventing the overcurrent.
Generally, in generating a command duty, which corresponds to a command voltage, for the inverter, it is necessary to detect the voltage of the DC circuit section to divide the command voltage by the detected voltage. A method of detecting the voltage of the DC circuit section has been known in which variation of a transient voltage rise of the DC circuit section entailed by a rapid deceleration control is removed by changing the time constant of the filter for detecting the voltage of the DC circuit section during the rapid deceleration control. Using the method, a higher voltage can be applied to the motor while keeping the amplitude of the command duty, thus achieving the rapid deceleration control (see Patent Document 3).